Samsung 245B+ - BN44-00173A - No backlight (24v)

[IU5ICI]

Hi all,
I have this monitor (Syncmaster 245B+) that turns on but doesn't have backlight.
Reading around, I replaced the 3 68ohm resistors but without luck.

Turning on the power supply (3.3kohm resistor between 5v and PS_on) with the lamp module connected I have 0V on 24V rail.
If I disconnect the lamp module I get the 24V but even with a 200ma load, the voltage drops to about 9v.

If I brutally connect an external 24V power supply directly on the 24V rail, it all works fine (No short, nothing getting hot) and the monitor works fine.

The caps don't seem bad.
I measured some outside the board:

CP803 - 78uf
CM802 - 17uf
CB876 - 1000uf
CB809 - 1000uf
CM851 - 1000uf

What else should I check ?
Thanks for the help.

P.s. the monitor stopped working from evening to morning. Shouted off fine and never came up again. (a couple years ago)

[CapLeaker]

Re: Samsung 245B+ - BN44-00173A - No backlight (24v)

I guess we need to have a high resolution, straight shot picture of your power supply to start with.

[IU5ICI]

Re: Samsung 245B+ - BN44-00173A - No backlight (24v)

Here they are. The illumination isn't the best, but you should still see.

In the last pic, the bridge between S_B and PSON is there just to turn on the power supply.
The 3 68ohm resistor are soldered badly (they ruined the PCB when they replace them but the solder is solid. They just need to be connected in series. No PCB need it the middle of the joint)

If you need something else, just let me know. Thanks

[budm]

Re: Samsung 245B+ - BN44-00173A - No backlight (24v)

Are you getting PFC Boosted Voltage? ICM801S may also be bad, need to check DCV on its pins, especially the VCC pin.
BTW, it is also in this thread:
https://www.badcaps.net/forum/showth...937#post715937

CP803 - 78uf
CM802 - 17uf
Are you sure? Those are odd numbers, also bad cap can have bad ESR but it will look normal.

[CapLeaker]

Re: Samsung 245B+ - BN44-00173A - No backlight (24v)

Picture #3 bottom right. There are 2 large caps next to the inductor. Those look like they run hot, as the sleeve shrunk a little (high ESR). Change them out. I believe it is CM862 and CM863. It should be a 10V 2200uF and a 35V 470uF. Those 2 caps don't look good to me at all.

[IU5ICI]

Re: Samsung 245B+ - BN44-00173A - No backlight (24v)

Are you getting PFC Boosted Voltage? ICM801S may also be bad, need to check DCV on its pins, especially the VCC pin.
BTW, it is also in this thread:
https://www.badcaps.net/forum/showth...937#post715937

CP803 - 78uf
CM802 - 17uf
Are you sure? Those are odd numbers, also bad cap can have bad ESR but it will look normal.

Honestly i don't know where i should check for PCF Voltage.
I made a new thread cause the problem was different.
And yes, you are right, CM802 is 17nf not uf. i should learn to read.

[QUOTE =CapLeaker]Picture #3 bottom right. There are 2 large caps next to the inductor. Those look like they run hot, as the sleeve shrunk a little (high ESR). Change them out. I believe it is CM862 and CM863. It should be a 10V 2200uF and a 35V 470uF. Those 2 caps don't look good to me at all.[/QUOTE]

Cm863 is 377.5uF, Vloss 1.4, 0.04ohm ESR and Cm862 is 1866uF, Vloss 2.2, 0.00 ESR
Since i should order some new caps, i'm going to replace them all and write if that fixed the problem.
If not, at least we sure know that caps aren't the problem..
For now, thanks for the help..

[CapLeaker]

Re: Samsung 245B+ - BN44-00173A - No backlight (24v)

CM863 is below 10% tolerance (377.5uF vs 470uF) so it is toast. I question your ESR meter at that point, it is not possible to have a 0.04Ohm ESR on that. The shrunken sleeves are a dead give away. Did you check them in or out of the circuit?

[IU5ICI]

Re: Samsung 245B+ - BN44-00173A - No backlight (24v)

My ESR meter is just a cheap chinese component tester.
But i can tell it isn't good at all.

[budm]

Re: Samsung 245B+ - BN44-00173A - No backlight (24v)

PFC Boosted Voltage is measured between the two legs of main filter cap in the primary side when the power switch is activated.

[IU5ICI]

Re: Samsung 245B+ - BN44-00173A - No backlight (24v)

PFC Boosted Voltage is measured between the two legs of main filter cap in the primary side when the power switch is activated.

Ok, thank you. When i'll get new caps, i'll check that.

I've removed all elecrolytic caps (to order new) and i read their values.
I'm pretty sure the ESR value is completely wrong.
At least the capacitance is about the same as two other DMM (different brands), so i "trust" that..

[cpt.charlie]

Re: Samsung 245B+ - BN44-00173A - No backlight (24v)

I would replace all caps with an ESR>1 ohm just in case.

[IU5ICI]

Re: Samsung 245B+ - BN44-00173A - No backlight (24v)

Ok, here I am again.

I replaced ALL electrolytic caps but the problem didn't change.
I still had 24v only without load: turning the PS on, the voltage was slowly rising to 24v but using just a 200ma load the voltage was dropping to 0.
Something got worse, since I don't have any voltage anymore, either without load: when I plug the PS, voltage slowly rise to .4-.5 volts then slowly drops to 0.

Across the big cap (CP803) I have about 390Vdc.
I checked diodes (DM851,DM852,DM854) but they are all fine.
I have 0 experience with switching PS and I honestly don't know what should I check next.

Any clue?

Thanks

[Andrew F. Ali]

Re: Samsung 245B+ - BN44-00173A - No backlight (24v)

Since you connected an independent 24V source to the board and every thing worked fine.
Trace the 24V backwards from the 'load point' to the Transformer, there must be some capacitor, resistor or diode not working properly. See what is connected to the copper trace, something may be loose or open. If nothing, then the transformer winding may be loose or increased impedance, but SMPS transformers don't usually go bad.

[budm]

Re: Samsung 245B+ - BN44-00173A - No backlight (24v)

So is the SMPS IC801S for the 24V circuit getting the VCC at pin 7 to run?
Check the solder joints around QB803 and verify the transistor and the ZENER are OK..

[IU5ICI]

Re: Samsung 245B+ - BN44-00173A - No backlight (24v)

So is the SMPS IC801S for the 24V circuit getting the VCC at pin 7 to run?
Check the solder joints around QB803 and verify the transistor and the ZENER are OK..

IC801S had 0 volt output. So i checked if Vcc was present on pin 6. There wasn't any voltage. I went checking QB802 but it was fine.
I don't know what happened next (probably i bridged something that souldn't be bridged) and both fuses blew up. I took out the bridge rectifier (thinking was the problem, since i have a short between + and -) but i still have a short.
I spent too much time on this and i think it's enough.
I have a couple 220/24v transformers laying around, so i'll probably just make a custom PS.

Anyway, thank you all for the help.